Frequency sensitive system



NOV. 25, 1941. K 'R FREQUENCY SENSITiVE SYSTEM 2 Sheets-Sheet i Filed April 5, 1940 FIG-3.1L

. INVENTOR. KARL RATH BY 5mg @ZJLA ATTORNEY.

Nov. 25, 1941. K. RATH 2,263,645

FREQUENCY SENSITIVE sYs'i'EM Filed April 5, 1940 2 Sheets-Shet 2 as 5 I8 stcowp 9.9 R. F. g'ggl DETECTOR- AMPuF AMPLIFIER AUDIO MIXER .JFIER 105 J04 6 1 0? as a C .109 410 L R F. F'RST I F. FREQUENCY H,- AUDIO =5 DETECTOR- LIMITER MODULATION .A' AMPLIFIER MIXER AMPLIFIER O DETECTOR AMPLIF|ER OSCILLATOR INVENTOR.

KARL. RATE-I ATTORNEY.

network is' resonant.

Patented Nov. 25, 1941 r reins FREQUENCY SENSITIVE SYSTEM Karl Bath, New York, N. Y., assignor to Radio Patents Corporation York , a corporation of New Application April 5, 1940, Serial No. 328,ll06

11 Claims.

The present invention relates to frequency variation response or sensitive systems of the typein which the magnitude, and possibly sign, of an output current or voltage depends on the departure of the frequency of an alternating signal or of the resonant frequency of a tuned circuit from some particular frequency.

Devices and circuits of this nature have various uses, for example for producing a tuning control potential in automatic tuning systems embodied in a radio receiving apparatus, in frequency control or. stabilizing arrangements in radio transmitters, and for demodulating frequenoy modulated carrier signals. Other uses are for translating or converting small mechanical movements affecting the capacity and/or inductance of a resonantcircuit so as to cause a detuning of the frequency of an auxiliary oscillator in electronic control apparatus and for various other applications as will become evi dent from the following disclosure.

The invention is more particularly concerned with frequency variation response systems of-the type comprising a phase shifting circuit, preferably a timed circuit for establishing at two points of this circuit alternating potentials differing in phase by an amount varying with the relative departure between the frequency of an alternating potential impressed upon the circult and the tuning frequency to which the cir cuit is resonant, and further means, for combining the said two potentials so as to obtain an electrical or mechanical output containing a component proportional to their product, that is,

in turn to said frequency departure to be detected or converted. In a commonly known arrangement for detecting or converting frequency variations, a

primary and a secondary electrical circuit, at

least one of which is tuned to a predetermined frequency, are coupled with each other inductively to form what is known as a band-pass filter network. If an alternating potential is impressed upon such a filter the phase difference between the potentials established at the high potential ends of the primary and secondary circuits varies proportionately to the relative frequency departure between the impressed signal and the tuning frequency to which the Thus, in a band-pass filter comprising two inductively coupled resonant circuits equally tuned to a predetermined frequency, the potentials at the high potential sides of the circuits will be shifted in phase by 90 relative to each other if the input frequency equals the tuning frequency of the circuits. This frequency departure will vary in either direction with respect to the normal phasedifference proportionately as the input frequency decreases below or increases beyond the tuning frequency of the circuits. The twopotentials of varying relative phase difference are combined such as by the aid of a balanced detector capable of producing anoutput' current or potential containing a component proportional to their product, that is, varying in amplitude proportionately to the relative phase difference between the impressed potentials and in turn to the relafive-frequency departure between the input signal and the tuning frequency to which the circuits are resonant.

Arrangements of the above and other known type have the disadvantage that due to the mutual coupling between the circuits constituting the phase shifting network their adjust:

,ment is critical and requires frequentrebalancquency detection or demodulation and to provide a novel system of the above character which is both simple in design and easy to adjust and balance and which will maintain its balance for a considerable length of time compared with the circuits known in the prior art.

Another object is to provide a frequency variation response circuit of the type mentioned hereinabove comprising at least two coupled circuits to establish potentials differing in phase proportionately to the relative frequency variations to be detected andwherein mutual reactance between the circuits is substantially eliminated and adjustment of either circuit enabled substantiallyindependently of the other circuit.

Further objects and advantages of the invention will become more apparent as the following detailed description proceeds taken with refer- 'ence to the accompanying drawings forming bodiment of the invention in a radio receiver to effect an automatic tuning control,

Figure 6 is a block diagram illustrating the embodiment of the invention as a detector or demodulator in a frequency modulation radio receiver.

Like reference numerals identify like parts in the different views-of the drawings.

With the above mentioned objects in view, the present invention contemplates the use of what is known as a variable space charge or virtual cathode in an electron discharge tube serving as a coupling element or impedance between circuit elements of a variable phase shifting system to produce potentials having a relative phase relation varying proportionately to the frequency variations to be detected or converted. If, in an electron discharge tube provided with a cathode, a control grid near said cathode, a plate or further grid and a screen grid located between said 2.

control grid and said plate and being maintained at a high positive potential with respect to said cathode, said plate or further grid is operated at cathode potential or "biased negatively with respect .to the cathode and an oscillating input potential is impressed upon said control grid, a concentrated electron space charge or virtual cathode will be formed in the space between said screen grid and said plate or further grid, said virtual cathode having a charge density varying in accordance with said oscillating potential. Said plate or further grid, according to the present invention, is biased and operated in such a manner that itcollects substantially no electrons and accordingly is purely capacitatively coupled with said variable space charge or virtual cathode in the manner described. This variable space charge which is due to 'a varying concentration of electrons which have passed the openings of the positive or screen grid and are retarded by the anode is equivalent to a fluctuating electron supply source and for this reason is referred to as a virtual cathode.

Such a variable space charge or virtual cathode induces a corresponding variable charge on said plate or further grid which gives rise to a displacement current in the plate circuit of the samefrequency as that of the oscillations impressed upon the control grid. The induced plate potential is displaced in phase by 90 rela- I.

tively to the grid alternating potential due to the capacitative coupling or generation of the plate tial.

In accordance with the present invention, capacitative or space. charge coupling of the above type is utilized to effect a varying phase shift between a pair of potentials derived from a resonant network and combined to produce an outa, put varying in intensity or amplitude proportionately to the frequency variations of an input potential or of the natural or resonant frequency of a tuned circuit or network.

Referring to Figure 1, there is shown a frequency sensitive circuit which may serve for converting an input signal of varying frequency impressed by Way of terminals w-b into a potential varying in amplitude proportionately to the input frequency variations derived from output terminals c-d. It is to be understood, however, that the invention is not limited to the conversion of frequency variations of a potential or current but applies equally to converting Variations of the natural or tuning frequency of a resonant circuit as will be explained in greater detail hereafter.

In Figure 1, the input alternating voltage which may be a high frequency signal of varying frequency is impressed by way of coupling coil l2 upon a resonant circuit comprised of an inductance I!) shunted by a condenser H and coupled to the grid l6 and cathode l5 of an electron tube [4 which comprises further a screen grid 11 maintained at a high positive potential with respect to the cathode in a manner well known and a plate IS. A further tuned circuit comprising an inductance 20 shunted by a condenser 2] is connected between the plate l8 and cathode IS in series with a source of potential 22, to provide a suitable negative steady bias for the plate 18 with respect to the cathode. The source 2| may be omitted and the negative plate bias produced solely by a resistance and condenser shunting network l5 in the cathode-toground lead. If desirable, the plate I8 may be at cathode potential to suit any existing requirement. As pointed out hereinabove, if the circuit 28, 2| is tuned to a frequency equal to that of the impressed alternating voltage a concentrated space charge or virtual cathode is formed adjacent to the plate l8 causing a potential to be induced on the plate of the frequency of and having a phase displaced by 90 with respect to the input potential applied to the grid l6. Thus, if the circuits l0-H' and 2il2l are equally tuned to a predetermined frequency and assuming that the input potential impressed by way of terminal ab is of the same frequency, the potentials at the high potential points A, B of the tuned circuits lill I and 202l will be exactly 90 out of phase and this phase difference will be increased or decreased, respectively, in proportion to the departure of the input signal frequency from the tuning frequency to which the circuits Iii-ll and Zil-ZI is resonant.

The potentials at the points A, B of varying relative phase are combined in a suitable manner to produce a common output including a component proportional to their product, that is, in turn to the frequency variations of the input potentials impressed by way of terminals a-b. In the example shown, the combining device takes the form of an electronic modulator or mixer tube 25 comprising a cathode 26, followed in the order named by a first control grid 21, a positive or screen grid 28 being maintained at a suitable positive potential with respect to the cathode, a second control grid 29 and an anode or plate 30. The potential established at the high potential side of the circuit I 0-H (point A) is impressed in the example shown .directls upon the first control grid 21 of tube 25, while the potential established at the plate H! or high potential side of the resonant circuit 262l (point B) is impressed upon the second control grid 29 of the tube 25 by way of an adjustable potentiometer resistance 23 and coupling condenser 24, the former serving to establish proper amplitude relation between the potentials impressed upon the grids 21 and 29 of the modulating or mixer tube 25. Item 24 is a leak resistance for the grid 29 and item 25 is a grid biasing network comprising a resistance-condenser shunt combination inserted in the cathode-to-ground lead of the tube.

There is thus produced in the output circuit of the tube 25 by reasonof the inter-modulation of the potentials on the grids 21 and 29 a component proportionate to the'product of the control potentials; thatis, in turn to the relative phase difference beween the potentials established at points A and B; This component varies therefore in amplitude proportionately to the frequency variations of the input potential impressed by way of terminals -19. This output component is segregated by the aid of a selective coupling device comprising a load impedance such as ohmic resistance 32 by-passed to ground by condenser 33 in the output circuit of the tube 25. The output potential may be derived from a portion of the load resistance 32 which is further by-passed by a condenser 34 applied to the output terminal c-d by way of a coupling condenser 35. The circuits I0-l| and 20--2l are advantageously carefully shielded from each other and from the rest of the apparatus by means of grounded screen indicated at and respectively.

It will beunderstood that a system shown in Figure 1 has the advantage over arrangements known in the prior art that the resonant circuits Ill-l l and 202l may be adjusted and balanced individually without mutual reaction, and that the balance will be maintained for a considerable length of time as a result of the uni-lateral coupling between the circuits in a direction from the input to the output of the tube M. The output coupling network 3233-34-35 shown in Figure 1 is designed fortranslating periodic variations such as sound or video currents in th case of a frequency modulated input radio signal comprising a carrier frequency varying in either direction according to the 'magnitudes I of the periodic modulating signals. If the frequency variations to be detected are of an aperiodic, i. e. a slow or progressive nature as in the case of automatic tuning control arrangements, the output coupling network is suitably modified to be responsive to variations of this character as shown more specifically in Figure 5 to be described later.

As pointed out the circuit described is well suited for demodulating signals of varying frequency impressed by way of input terminals a-b. According to a further embodiment of the invention the circuit may also serve for converting variations of the resonant frequency of a tuned circuit, preferably the circuit '20--2l into corresponding amplitude variations of an output current or potential supplied at terminals cd. In this case an alternating potential of fixed frequency'isimpressed upon input terminative method of combining the potentials established at points A, B to produce output energy having a frequency responsive amplitude in accordance with the invention. In the example illustrated an aperiodic input circuit is provided by the' omission of the condenser I I, the remaining elements associated with the tube l4 being substantially similar to those of Figure 1. .In order to combine the potentials established at points A and B having a varying relative phase relation thereis provided a ring modulator or rectifier bridge circuit adapted to produce output energy having an amplitude proportional to the product of a pair of input potentials and comprising four rectifying elements of any suitable type such as dry rectifiers 36, 31, 38, 39. The rectifiers are connected to form a closed circuit and arranged in like sense as regards their current passing directions. One of the voltages to be compared and established between point A and ground is impressed upon one pair of diagonal points or apices of the bridge circuit by way of a coupling coil 40 arranged in inductive relation with the input coupling inductance III, while the other voltage established between point B and ground is impressed upon the remaining apices of the bridge circuit by way of coupling coil 4| arranged in inductive relation with the inductance 20 of the resonant circuit. The output is derived from suitable points preferably the mid points of the induction coils 40, 41 and applied to output terminals cd by way of a transformer 42. A pair of choke coils 43 and 44 are advantageously inserted in the output leads sponding amplitude variations of electrical ento prevent high frequency input currents from directly passing to the output circuit.

A further modification of an inventive system is shown in Figur 3 which embodies a further vacuum tube as a phase inverter to produce an additional potential (at point C) having a phase opposite to the phase of the input potential (at point A). By combining the quadrature potential established at point B with either of the potentials at points A and C sum and difference potentials are obtained which latter are rectified and combined differentially to produce an output varying in accordance with the product of the potentials at points A and B and including a frequency responsive component in a manner similar to that described hereinbefore. In the example shown a composite vacuum tube is provided comprising a pentode section associated with the variable phase shift circuit and a triode section to serve as a phase inverter. The tube 50 provided for this purpose includes a common cathode 5|, a pentode section comprising an input grid 52, a positive or screen grid 53 main tained at a high positive potential with respect to the cathode, and a suppressor grid 54 directly tied to the plate 55, and a triode section comprising the cathode 5l, a control grid 56 and an anode 51. Both sections are suitably shielded from each other such as by means of a grounded screen in a manner well understood. Item 58 is a biasing network comprising a resistance bypassedby a condenser and inserted in the oathode-to-grcund lead of the tube. The plate '55 of the pentode section is connected to ground through a tuned circuit comprising an inductance 6B shunted by a condenser BI and suitably shielded from the remaining parts of the system by means of a grounded screen 60'. In this manner the pentode section of the tube provides a space charge coupling between the input circuit and the circuit Elk-6i thereby establishing a potential. on" the plate. (pcintrB); having: a phase varying relative: to. the phase; of: the: input. p;- tential (point A) in: proportion to: the: relative frequency: departure. of: the impressed potential from the tuning frequency; to; which; the. Elk-6i: isi resonant in; the manner; described in detail hereinabove.

The grid 56' ofthextniode section isacoupledtin theiexampleshown, directly-to the-high potential side. of: the input circuit: (point-A0; and: the: plate of thetriode section is connected: to the plus pole. of aspace current source throughxa-a coupling impedance preferably an ohmic: resistance; 68.. Thereiis: established; in: this manner 0111131118? plate 5.1 (:point'C) a potential having a. phase opposite toithe phaseoftheihput. potentiaL applied. to grid 52; There are further: shown: irrE-igure 3i apair of: doubleztriode tubes 62: and: 63 serving; for; combining the; potential at point. B: with either'off the potentials at point A. and point: C or; in. other words; to produce sum; and differencepotentials fromthepotentials at points A and B:and:.to.=rectifiy and to. differentially combine the: thus. obtained sum and. differencepotenfials to' produce a. frequency responsive output. For-this? purpose pointsA', Band C are connected withlthe; double trioder tubes 82 and 63 having controligridszdfi, 65 and 66', 6.1.", respectively,. as followszz Point? B is connected to grids64: and-161: of one: oi thetri'ode sectionsof eaohtube, point A. isconnected to the grid 65 of the. remaining. triode section of tube fit while point C is coupled: tolthe grid 65: of the remaining triode section: of tube 62-; by way of coupling: condenser 6:1 and. grid! leak resistance 68; The plates of the tubesgare tieditogether and ner; across; theterminal c-d the: differential. of 4 the rectifiedsum' and difference of'the potentials established at. points A and. B. This difierential potential. will include a componentvarying proportionately to the: relative. phase: b'eweenzthe potentialsxat points-ArandB; that is; in turn toL'the relative departure of the frequency of the input potential from thetuning frequency to. which the: circuits: 60-45 1 is resonant.

Referring. toFigureA', there is shownstill an.-

otlier: modification. of. a": frequency. sensitive sysitern according tothe invention. According to this exemplifi'cation. the input. signals of varying orfi'xed frequency. are impressed uponthe grids 52; and. 58 of? the pento'de and" triode sections of. a composite: tube 50 in substantially the U same manner'a sishown. in Figure 3;.whereby potentials. will be established at points: B and C whose relative phase varies proportionately to the variations of the input frequency orv the variations of the resonant frequency of the circuits 60'6l as the case' may be; Inthe present example the potentials established at points B and" C are impressed upon the control grids l2 and 1'42 of a. composite pentod'e diode tube lfl'by way of coupling condensers 82 and Bil and grid leak. resistances 83 and: 81, respectively. The tube further comprises a commoncathc'de- TI, a. main anode or' plate: a: diode plate 16' arranged to cooperate with the cathode 1 I, and: a positive-or screen grid-.13 arranged" between the control grids l2 andil t and. maintained at'a suitable: positive potential relative to the cathode. Item His a grid biasing network comprisinga resistance shunted by a condenser and inserted in the cathode-to-ground lead. The plate 15 is 75 connected. to the plus pole; of a space current source through a tuned resonant circuit. comprising an; inductance coil 8.4- shuntedzby apondenser 85. and. a resistance. 8.6. This: resonant circuitis in turn coupleditoz a; secondary: resonant circuit. comprising an: inductance 8 1: shunted by a. condenser 88, whereby both; circuitsform: a band-pass coupling, network: suitably shielded fromthe remaining partsoi the'systemxby, means of a screen 84!. The-purpose ofthe resistance inshunting. the resonant circuit 84'-85ais:to:sufficiently broaden the. frequency response: to. encompass thewhole range: of. frequency: variations impressedupon the input: a;-b in. case of receivinga. wide .band frequency modulated: signal;

In an arrangement of this typetthe frequency modulated signals are converted intocorrespondiing, amplitude modulated. signals. in the: circuit 84;88 by the combinediaction ofthe'grids: 'M' and 7 2;. The thus obtained; amplitude modulated: signal isimpressed from the circuitr81;-88 uponthe diode plate 16. whereby a. frequency'responsive, or demodulated potentiali is; produced across" the diode load. resistance 88 shuntedbyra. smoothing condenser 89 and connected across output terminals c.d; in a manner described in detail in my: co-pendingpatent application; Sen. No; 318;.- 9572; filed; February; 14, 1940 entitled Frequency variation response circuit. which matured into Patent 2,248,197- issued. J ulyr 8; 19.41.

Referring to Figure 5, there. is. shown a: radio receiving system in. block diagram form embodying a frequency responsive system according to. theinvention serving as a discriminator for an automatic tuning arrangement incorporated in the receiver. Radio signals received? by an antenna 9B areimpressed. uponia radio frequency amplifier 94 byway of a coupling transformer 93. Theamplified: radio: frequency signals are. combined in afirst detector ormixedi with signals of different frequency generated" by a; local: oscillator 96 to produce intermediate frequency signals which are efiectively and selectively amplified inian'intermediate' frequency amplifier 91. The amplified. intermediate frequency: signals are appliedto a second detector and; audio amplifier 98 supplying an output energizing a suitable translating device such as a loud speaker 95. Fractional. intermediate frequency energy is derived by way of terminals: (r -b and impressed uponthe frequency responsive. system which. in the exe ample shown is ofv substantially the same. type as illustratedi in" Figure 1. with the exception" of thewoutputcoupling network for thetube 25;. The latter in: the example shown comprises a coupling resistance 32 in series with the. space: current source 3-2 and a. condenser shunted. between ground and the plate. of tube 25. Such a couplingwillbe responsive to aperiodic'and' slow variations proportional to thedeparture between. the frequency of. the intermediate signals and the tuning frequency of the resonant circuit -21 which. latter'in. this case is tuned to the intermediate frequency of: the amplifier: 91? for which the system. is designed; The tune detecting potential thus obtained. across terminals c'd is impressed upon a suitable tuning control element of the local oscillatorrSB- such asupon the grid-10f an electronic. reactance tube controlling the frequency of the local oscillating. tankcircuit in such a manner as to' maintain the. de- 'parture. between thesignal frequency and". the intermediate frequency of the: receiver within a desired minimum range.

Referring. to'Figure' 6 there isshown in block I claims.

cuit comprising an inductance IIlI shunted by a condenser I02 and a band spread resistance I03.

After amplification in a radio frequency ampli-.

fier I04, the radio frequency signals are converted into signals of intermediate frequency in a first detector-mixer stage I06 having associated therewith a local oscillator I05. The intermediate frequency signals are amplified in an intermediate frequency amplifier, I01 and thereafter freed from any spurious amplitude modulation in a limiter I08 of known construction. The pure frequency modulated signals obtained from the output of the limiter I08 are impressed upon input terminals ab of a frequency modulation detector which may consist of any one of the systems described hereinabove and serves to demodulate the frequency modulation representing the intelligence being transmitted into a corresponding output signal to be further amplified in an audio amplifier I I19 and applied to a suitable translatin device such as a loud speaker III].

It will be evident from theforegoing that the invention is not limited to the specific circuits and arrangement of parts disclosed and described herein for illustration but that numerous variations and modifications may be resorted to as will suggest themselves to those skilled in the art, all coming within the broad scope and spirit of the invention as defined in the appended Thus, for instance, the combinationof the phase shifted potentials established at points A and B may be effected purelymechanically by applying potentials or currents derived from points A and B, to different coils of an electrodynamometer. The force of attraction between these coils is then proportional to the product of the potentials and accordingly will vary in the required manner with the relative departure of the input frequency from the tuning frequency of the resonant circuit and may be adapted for any suitable purpose such as for moving a. pointer or operating a relay or the like. The specification and drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A frequency variation response circuit comprising a source of alternating current, resonant impedance means, the relative frequency of said alternating current with respect to the resonant frequency of said impedance means being variable, an electron dischargetube comprising at least a cathode, a control grid, a screen grid and a further electrode, all arranged substantially in the order named, means for impressing potential from said source upon said control grid, means for maintaining said screen grid at a positive steady potential with respect to said cathode, further means for maintaining said electrode at a relatively low steady potential with respect to said screen grid to substantially prevent a steady electron current toand to bring about a variable electron space charge in the vicinity of said further electrode, means for coupling said impedance means to said electrode and cathode, separate means exterior of said tube for combining the alternating potentials at said control grid and said electrode to produce output energy proportional to their product, and further means forsegregating from said output energy a component varying proportionately to the' relative frequency departure of said im-,

pressed potential from the resonant frequency of said impedance means. 7

2. A frequency variation response circuit comprising a source of high frequency current, resonant impedance means, the relative frequency of said current with regard to the resonant frequency of said impedance means being variable, an electron discharge tube comprising at least a cathode, a control grid, a screen grid and further electrode, all arranged substantially in the order named, means for impressing potential from said source upon said control grid, means for maintaining said screen grid at a positive steady potential with respect to said cathode, further means for maintaining said electrode at a negative steady potential with respect to said cathode to substantially prevent a steady electron current to and to bring about a variable space charge in the vicinity of said electrode, means for coupling said impedance means to said electrode and cathode, separate means exterior of said tube for combining the high frequency potentials at said control grid and said electrode to produce output energy proportional to their product, and further means for segregating from said output energy a component varying proportionately to the relative frequency departure of said impressed potential from the resonant frequency of said impedance means.

3. A frequency variation response circuit comprising a source of high frequency current, resonant impedance means, the relative frequency of said source with respect to the tuning frequency of said resonant impedance means being varied, a first electron discharge tube comprising at least a cathode, a control grid, a screen grid and a further electrode, means for impressing potential from said source upon said control grid, means for maintaining said screen grid at a steady high positive potential wth respect to said cathode, further means whereby said electrode is maintained at a relatively low steady potential with respect to said screen grid to substantially prevent electrons from being conveyed to said electrode and to bring about a variable space charge adjacent to said electrode, means for coupling said resonant impedance means to said electrode and cathode, a second electron tube comprising at least a cathode, a pair of control grids, a screen grid arranged between said con trol grids and an anode, means for maintaining the screen grid and anode of said second electron tube at positive steady potentials with respect to the cathode, means for impressing each of the high frequency potentials at said control grid and electrode, respectively, of said first electron tube upon one of the control grids of said second electron tube, and coupling impedance means in the anode circuit of said second electron tube effective in developing output energy having an amplitude varying proportionately to the relative phase variations between said control potentials.

e. A frequency variation response circuit com prising a source of high frequency current, resonant impedancemeans, the relative frequency of said source with respect to the tuning frequency of said resonant impedance means being varied, an electron discharge tube comprising at least a cathode, a control grid, a screen grid and a further electrode, means for impressing potential from said source upon said control grid, means for maintaining said screen grid at a relatively high steady positive potential with,

respect to said: cathode, means, for maintaining said electrode at a relatively low steady potential with respect to said screen grid to, substantially prevent electrons from being conveyed to said electrode and to, bring about a variable space charge-adjacent to said electrode, means for coupling said resonant impedance means. to said electrode and cathode, a modulator comprising four rectifying elements arranged in like sense to form a bridge circuit, first circuit means for impressing a first high frequency voltage established between said grid and cathode upon one pair of apices of said bridge circuit, second circuitmeans for impressing a second high frequencyvoltage established between said electrode and cathode upon the remaining pair of apices of said bridge circuit, an output circuit connected between tapping points of saidfirst and second, circuit means, and coupling impedancemeans in said output circuit effiective in develop.-- ing output energy having an amplitude varying proportionately to the relative phase variations between said first and second high frequency voltage.

5. A-frequency variation response circuit comprising a source of high frequency current, resonan-t impedance means, the relative frequency of said source with respect to the tuning frequency of said resonant impedance means. beingvaried, electron dischargetube comprisingat least a cathode, a control grid, a. screen'grid and a further electrode, means for coupling said. source'tosaid control grid andcathode, means for maintaining said screen grid ata relatively steady highpositive potential with respect to saidcathode, further means for maintaining said electrode at a relativelylow steady potential with respect to said screen grid to substantially prevent electronsfrom being conveyed to said electrode and to bring about a variable space charge adjacent to said electrode, means for coupling said resonant impedance means to saidelectrode and cathode, circuit means for producing sum anddiiference potentials from the potentials established at said: control grid and said electrode, respectively, means for rectifying saidsum and d-i-lrerence potentials, and further means for differentially combining the rectified potentials.

'6. A frequency'variation response circuit comprising a source of high frequency current, resonant impedance means, the relative frequency of said source with respect to the tuningfrequency of said resonant impedance means'being varied, a first electron discharge tube comprising at least a cathode, a control grid, a screen grid and a further electrode, means for coupling said source to said control grid and cathode, means for maintaining said screen grid at a relatively high steadypositive potential with respect to said" cathode, further means for maintaining said electrode at a relatively loWsteaoly potential with respect to said screen grid to substantially prevent electrons from being conveyed tosaid electrode and to bring about a variable space charge adjacent to said electrode, means forcoupling said resonant impedance means to said electrode and cathode, a second electron tube having at least a cathode, a control grid and an anode, means for impressing potential from said source upon the controlgrid of said second electron tube, circuit means'for combining the potential established at. said electrodeof said first elec-' tron; tube with either of the potentials established at the grid of. said first. electron tube and attheanode of saidseqond. electron, tube, means for rectifying the combined potentials, and fLur-v ther means for differentially combining the rec.-

tifieol potentials.

A frequency variation response circuit comprising a source 03 high frequency current hay ing a varying frequency, a tuned circuit resonant to a predetermined fixed frequency, an electron discharge tube comprising at least: a cathode, a control grid, a screen grid. and a further.

electrode, all arranged substantially in the order named, means. tor impressing potential from said source upon said control grid, means for main-. taining said screen. grid: at a steady positive potential with respect to said cathode, further means for maintaining said electrode at a relatively low steady potential with respect to. said screen grid to substantially prevent a steadyelectron current to and to bring about a variablespace charge in the vicinity of' said electrode, means for coupling said tuned circuit. to. said.- electrode and cathode, separate means exterior of said tube for combining the high frequency potentials, upon said control grid and said elec-. trode to produce output energy proportional to their product, and further means for segregatingfrom said output energy a component having an amplitude varying proportionately tothe relative frequency departure, of said impressed potential from theresonantfrequency of said tuned circuit.

8. A frequency variation response circuit comprising: a source of frequency modulated signals, a fixedly tuned; circuit resonant to the center frequency of said signals, an electron discharge tubecomprisingatl'east a cathode, a control grid,

a screen grid and; further electrode, all substantially in the order named, means for impressing signal potential: from said source upon said control grid, means for maintaining said screen grid at a steady positive: potential with respect to said cathode, further means for maintaining said electrode at a relatively low steady potential with respect. to said screen grid to substantially prevent a steady electron current to and tobring about; a variable spacecharge in the vicinity of said: electrode, said tuned circuit being con-r nected to said electrode and cathode, separate means. exterior of said tube for combining the high frequency potentials at said control grid and said electrode to produce output energyproportional totheir product, and impedance means to segregate from said output energy a, component having an amplitude varying proportionately to the frequency variations of said signals.

9. A frequency variation response circuit comprising a source of alternating current having a substantially fixed frequency, a tuned circuit, the resonant frequency of said tuned circuit being variable, an. electron discharge tube comprising at least a cathode, a. control grid, a screen grid and a further electrode, all arranged substantially in the order named, means for impressing potential from said source upon said control grid, means for maintaining saidscreen gridat a steady positive potential with respect to said cathode, further means for maintaining said electrode at a relatively low steady potential with respect to said screen grid tosubstantiallyprevent a steady electron. current to and to bring about. a variable space charge the vicinity of said electrodasaid tuned circuit bein connected to said electrode and cathode, separate means exterior of said tube for combining; thev alterratin p entia s at aid: control d and sai electrode to produce output: energyproportional to their product, and further means for segregating from said output energy a component having an amplitude varying proportionately to the variations of the resonant frequency of said circuit.

10. A frequency variation response circuit comprising an input circuit carrying high frequency currents, an electron discharge tube having means for producing an electron space current, means including circuit connections from said input circuit to said tube for varying said space current in accordance with said high frequency currents, an electrode in said tube with means for applying thereto a steady bias potential whereby substantially no electrons are conveyed to said electrode from said space current and a concentrated electron space charge is set up adjacent to said electrode, resonant impedance means, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant impedance means being 11. A frequency variation response circuit comprising an input circuit carrying high frequency currents, an electron discharge tube having means for producing an electron space current. means including circuit connections from said input circuit to said tube for varying said space current in accordance with said high frequency currents, an electrode in said tube with means for applying thereto a steady negative bias potential whereby substantially no electrons are conveyed to said electrode from said space current and a concentrated electron space charge is set up adjacent to said electrode, a resonant circuit, the relative frequency of said high frequency currents with respect to the tuning frequency of said resonant circuit being varied, means for connecting said resonant circuit to said electrode to produce a displacement current therethrough by coupling with said space charge, means for deriving a pair of control potentials from said input circuit and said resonant circuit having a relative phase varying according to the relative frequency departure of said high frequency currents from the tuning frequency of said resonant circuit, further means for producing output energy having an amplitude proportional to the product of said control potentials, and means for deriving a component from said output energy varying in amplitude proportionately to said frequency departure. 

